On a hot coil production line at an electric furnace minimill, for example, high frequency induction heating using high frequency electric power of high energy density has widely been used conventionally to heat a thin slab (a kind of hot rolled steels) produced by continuous casting. FIGS. 8 and 9 show an induction heating apparatus 20 that has been generally used to heat a thin slab on a continuous production line. This apparatus 20 has a construction such that a thin slab 21, which is continuously cast in a continuous casting section outside the figure and is supplied to a heating section, is subjected to induction heating (high frequency heating during movement) under a moving condition.
As shown in FIGS. 8 and 9, the induction heating apparatus 20 is made up of a plurality of steel-made conveying rollers 23 arranged at intervals along a predetermined conveyance path, a solenoid-type induction heating coil 22 fixedly arranged between the adjacent conveying rollers 23, and a high frequency power source 24 for supplying high frequency electric power to the induction heating coil 22. The aforementioned induction heating coil 22 used as heating means is a solenoid-type coil wound a plurality of turns in a spiral form. Specifically, as shown in FIGS. 8 to 10, the induction heating coil 22 is formed by the repetition of a configuration of one turn consisting of a lower winding portion 22a, a side winding portion 22b rising upward from one end of the lower winding portion 22a, an upper winding portion 22c connecting with the upper end of the side winding portion 22b, and a side winding portion 22d falling downward from one end of the upper winding portion 22c.
Thus, a thin slab 21 is placed on the plurality of conveying rollers 23 and conveyed so as to pass through a hollow portion (a portion surrounded by coil winding) of the solenoid-type induction heating coil 22. More specifically, the thin slab 21, which is supplied continuously from the continuous casting section outside the figure, is placed on the plurality of rollers 23, which are rotated at an equal speed in the same direction, and is conveyed in a predetermined direction (in the direction of the arrow mark X in FIGS. 8 and 9). At this time, high frequency electric power of the high frequency power source 24 is transmitted to the thin slab 21, which is a heated body, by means of the induction heating coil 22, whereby the thin slab 21 is heated to a predetermined temperature by high frequency induction heating during the movement. In this case, the conveying speed of the thin slab 21, the rotational speed of the conveying roller 23, and the high frequency electric power of the high frequency power source 24 are controlled in accordance with the type of the thin slab 21, by which the heating temperature of the thin slab 21 is controlled.
In order to efficiently heat both of the upper and lower surfaces of the thin slab (heated body) 21 with a thickness of about 20 to 30 mm and a width of about 1000 to 1400 mm, the shape of an opening portion 25 of the induction heating coil 22, that is, a coil shape viewed from a plane perpendicular to a coil axis S.sub.1 is made rectangular, and the area of the opening portion 25 is determined so as to be at a necessary minimum. The axis S.sub.1 of the induction heating coil 22 is arranged so as to be substantially in alignment with the axis S.sub.2 of the thin slab 21 (see FIG. 9).
The induction heating coil 22 is excited by the high frequency power source 24, and the frequency of the high frequency power source 24 is set at about 5 to 6 KHz so that the penetration depth of induced current is not larger than a half of the thickness of the thin slab 21. An electromagnetic field (magnetic flux) generated by the induction heating coil 22 produces an eddy current in the thin slab 21. Taking the eddy current as I and the electric resistance of the thin slab 21 as R, Joule heat of I.sup.2 R is produced, so that the temperature of the thin slab 21 increases. A higher heating electric power is more effective in increasing the productivity of minimill and in shortening the production line. Therefore, with the high-power high frequency power source 24 of 1000 to 2000 KW, which is the highest class that can be achieved by the present-day technology, and the induction heating coil 22 being one set, several sets to ten and over sets are arranged in series in the conveying direction of thin slab, thereby forming one heating line.
However, the induction heating coil 22 produces a slightly but non-negligible, harmful eccentric magnetic flux in addition to a magnetic flux parallel with the coil axis S.sub.1, which is effective in heating the thin slab 21. This eccentric magnetic flux is generally caused by the coil winding that is wound in a spiral form while shifting in the direction along the coil axis S.sub.1, that is, the coil winding that is wound at a predetermined lead angle .theta. (see FIG. 10) in the solenoid-type induction heating coil 22. In this case, the lead angle is an angle formed between a line S.sub.3 in the direction perpendicular to the coil axis S.sub.1 (a line in the direction agreeing with the coil width direction and the width direction of the thin slab 21) and the upper winding portion 22c of the induction heating coil 22 as shown in FIG. 10. Taking the lead angle as .theta., cos .theta. is an effective component, and sin .theta. is a component that produces the eccentric magnetic flux. In an example in which the opening size of the opening portion 25 of the induction heating coil 22 is 1600 mm.times.110 mm, the depth size is 280 mm, and the winding material is a copper pipe of 50 mm.times.30 mm, the lead angle .theta. is about 1.degree..
FIG. 11 shows induced current components produced on the upper surface of the thin slab 21 by electromagnetic induction caused by the induction heating coil 22 wound so as to have the lead angle .theta.. As shown in FIG. 11, on the upper surface and in the vicinity thereof of the thin slab 21, an induced current i.sub.0 flows in the direction along the upper winding portion 22c. In this case, an induced current component i.sub.1 =i.sub.0 cos .theta. flowing in the width direction of the thin slab 21 is produced as a component effectively contributing to the induction heating of the thin slab 21, and on the other hand, an induced current component i.sub.2 =i.sub.0 sin .theta. flowing in the direction of the axis S.sub.2 of the thin slab 21 (or the direction of the axis S.sub.1 of the induction heating coil 22) is produced as a component harmful to the induction heating of the thin slab 21. That is to say, if the eccentric magnetic flux is present, the induced current component i.sub.2 flowing in the axial direction of the thin slab 21 is produced (see FIGS. 8 and 11).
If the induced current component i.sub.2 flowing in the direction of the axis S.sub.2 of the thin slab 21 is produced in this manner, an axial current i.sub.2 indicated by the broken line in FIG. 8 passes through a conveying roller 23b, which is disposed on the downstream side in the thin slab conveying direction with respect to the induction heating coil 22, and a ground line G, reaches a conveying roller 23a disposed on the upstream side in the thin slab conveying direction with respect to the induction heating coil 22, and returns to the thin slab 21, the axial current i.sub.2 being a circulating current that circulates along the loop. As a result, by this circulating current, a spark (arc) is produced between the thin slab 21 and the conveying roller 23a and between the thin slab 21 and the conveying roller 23b, so that the back surface of the thin slab 21 arranged corresponding to the conveying rollers 23a and 23b, especially the side edge portion of the back surface thereof, is damaged greatly by overheat caused by the spark, and also the surfaces of the conveying rollers 23a and 23b are electrolytically corroded. The lead angle of the coil winding is not zero depending on the winding construction even if the mechanical lead angle .theta. of the coil winding with respect to the thin slab width direction shown in FIG. 10 is zero. This is because for the single-layer, multi-wound solenoid-type coil, an axial current component is always present in accordance with the size in the depth direction.
Accordingly, as the most general countermeasures against the occurrence of the axial current i.sub.2 as described before and against the damage and electrolytic corrosion of the thin slab 21, a method in which the plurality of rollers 23 are insulated from the ground line (earth potential) has been used conventionally. However, this method has a problem in that each of the conveying rollers 23 must be insulated, so that the equipment becomes complicated and expensive. As an alternative, the conveying rollers 23 may be made of a ceramic material. In this case, the ceramic roller is high in cost, and is easily scraped or cracked, so that a problem of durability is actually presented. Further, as other countermeasures, various methods have been tried, such as a method in which the conveying roller 23 formed by ceramic coating the surface of a stainless steel roller is used, or a method in which a base for supporting the shaft of the conveying roller 23 is insulated from the ground line. However, all of these methods are dissatisfactory in terms of ease of manufacture, price, and durability of equipment.
Also, as the conventional alternative countermeasures against the occurrence of the axial current i.sub.2, a method is sometimes used, in which as shown in FIG. 9, an iron core 30 formed by laminating silicon steel plates is disposed around induction heating coils 22 so that the whole or part of magnetic path generated on the outside of the coils 22 is covered by the iron core 30. In this case, the direction of the plane of the silicon steel plate is made in parallel with the magnetic flux in the direction of the coil axis S.sub.1, by which the magnetic flux at right angles to the direction of the coil axis S.sub.1 is shut off by the iron core 30. However, this method is dissatisfactory because the construction for cooling and supporting the iron core 30 is very complex, so that there is difficulty in manufacturing and the price is very high, especially in the equipment of high electric power.
The present invention has been made in view of the above-described actual situation of the prior art, and accordingly an object thereof is to provide an induction heating coil, in which the occurrence of a circulating current (a circulating current causing a spark produced on a contact face between the heated body and the conveying roller) harmful to induction heating, which flows circularly in a heated body such as a thin slab and conveying rollers can be prevented by contriving the way of winding of the induction heating coil, and therefore the damage to the heated body caused by the circulating current flowing in the heated body along the coil axis direction and the electrolytic corrosion of the conveying roller can be prevented, and an induction heating apparatus using this coil.